The present invention relates in general to transmissions for automatically changing speeds of vehicles or machinery and, more particularly, to a new design for a greatly simplified automatic transmission which can be used in a wide variety of applications including bicycles, lawn and garden equipment, tractors, motorcycles, automobiles, and other machinery for effecting automatic speed changes as power is transmitted to a load which is driven manually, by an electric motor or by an internal combustion engine. More particularly, a fixed diameter drive wheel affixed to a drive shaft is coupled to a fixed diameter driven wheel affixed to a driven shaft by an elastic drive belt for automatic changes in speed of the drive shaft relative to the driven shaft as power is transmitted between the two shafts.
Conventionally, speed changes in machinery are performed by transmissions wherein the changes are performed by changing radius ratios of drive wheels and driven wheels, i.e., pulleys, sprockets, gears and the like. Changing the radius ratios increases or decreases the speed of a driven wheel for a given speed of the drive wheel and, conversely, decreases or increases the torque applied to the driven wheel. Due to varying load conditions, it is often desirable to adjust the radius or speed ratios so that the torque load on the power source, whether a person, motor or engine, is within the capacity for efficient operation of the power source. One such situation exists for acceleration of a vehicle to a desired speed from a standing start.
Manual transmissions permit changes of radius ratios or gears to accomplish speed/torque control. Manual transmissions permit an operator of a machine to manually select the radius ratios or gears and permit operators to control operating speeds of a machine manually. Manual transmissions, while less popular in recent years, are still common in motor vehicles, particularly in trucks, and are almost universal in bicycles where the most common gear shifters comprise derailleur systems which move a chain among a series of two or more adjacent sprockets.
The most popular transmission in motor vehicles in the United States is the automatic transmission which automatically shifts gears within the transmission. The automatic transmission includes three major components: a torque converter, at least one planetary gear system, and a hydraulic control system. The torque converter is a hydraulic coupler composed of an impeller or pump and a turbine, plus a stator to multiply the torque. The torque converter is essential to the automatic transmission because it provides smooth clutching between the engine and the transmission gears. However, it is also the main source of inefficiency within the transmission.
A planetary gear system is composed of a sun gear, planet-pinions and an internal gear. The planetary gear system can provide an increase in speed with a decrease in torque, a decrease in speed with an increase in torque, or provide for changes between reverse, neutral and direct drive by locking one or more gearing members. More than one planetary gear set is often used in an automatic transmission to provide additional speed ratios. While gears are the most efficient power transmission elements, they have high precision requirements, are expensive to manufacture and generate noise in operation.
The hydraulic control system of a conventional automatic transmission comprises numerous components such as governors, servo bands, clutches, check valves, balance valves, modulator valves, pressure regulator valves and the like to control the transmission in response to sensed speed and throttle pressure. The hydraulic control system controls the shifting of the gears by either locking one or more of the sun gears or planet-pinions, or by activating servo bands or clutches. Thus, the hydraulic control system adds to the complications and expense of the conventional automatic transmission used in today's motor vehicles. In addition to the noted complexities, conventional automatic transmissions produce substantial amounts of heat and must be cooled for proper operation.
In summary, conventional automatic transmissions as used in many modern day motor vehicles are complicated, expensive, require substantial cooling and, upon failure, are expensive to repair.
Another form of automatic transmission is the variable-speed drive. Variable-speed drives provide an infinite number of speed ratios within a specific range, and may be made in the form of a cone drive, disk drive or belt drive. The belt drive is the most common variable-speed drive design with the variable speed ratios being achieved by changing the effective diameters of two pulleys. The pulleys are made of two flanges to fit a V-belt. One flange is fixed, and the other flange is adjustable in an axial direction. The separation of the two flanges changes the effective diameter of the pulley to the V-belt. While one pulley opens the flanges to reduce the effective diameter of that pulley, the other pulley pulls the flanges closer together to increase the effective diameter of that pulley. Then the speed ratio of the two pulleys reflects the new ratio of the effective diameters.
Some automatic transmissions using variable-speed drives have been developed and used on motorcycles, garden and farm equipment, as well as some small automobiles. In operation, the variable-speed drive automatic transmissions adjust the pulley widening and closing by electrical, hydraulic or mechanical means. In general, the torque or speed of a driven or driving pulley is sensed and then the flange gaps of the two pulleys are synchronously adjusted. Unfortunately, variable-speed drives primarily rely on friction between the pulley and the V-belt such that frictional losses are unavoidable, the tension of the belt must be accurately controlled by springs or linkage -controlled pulleys, there is a limitation of load capacity and slippage can not be completely avoided. Accordingly, variable-speed drives do not present a viable solution to the problems of conventional automatic transmissions.
In all of the above-described currently-known transmission and drive systems that utilize pulleys or belts, the belts employed therein are, and are designed to be, substantially non-elastic. (As used herein, "belts" include not only rubber and synthetic belts, but also cables (e.g., spring steel cables), ropes, etc., and any other material that may form a loop for use with a pulley system). That is, the belts of the prior art transmissions are designed to stretch lengthwise to only a minimal degree upon application of tension--i.e., these belts generally have a very high spring constant `k.` While all belts will inherently possess some longitudinal expansibility--e.g., the natural expansibility of a rubber belt--in the designs of prior art transmission systems it is necessary and desirable to limit strictly this stretching capacity. One reason for strictly controlling the elasticity of transmission or drive system belts in the prior art is that an overly-strechable belt would introduce unwanted slack into the transmission system, disrupting the transmission of power. For instance, a belt made slack by stretching under tension--even if the stretching is only transient and the belt regains its former conformation upon cessation of the tension--may not make proper contact with the driving wheel or the driven wheel of the transmission, due to slippage or other undesirable effects. Further, the gearing ratio of prior art transmissions would be disrupted by speed changes introduced by belts whose length could change substantially under tension, as this length change would introduce differential wheel speeds (as set forth hereinbelow, in the present invention such elongation-induced differential speeds are a desirable and predictable result of the present design, but in the prior art would represent unpredictable and undesirable side effects). Accordingly, while prior art transmission systems necessarily tolerated a de minimis amount of elasticity in belts, the elongation associated with this elasticity is typically less than one percent (1%) under the maximum tension that was applied during use of the transmission system (i.e., the transmission belts of the prior art would stretch to a length no greater than one-hundred-and-one percent (101%) of their original length). A ten percent (10%) maximum stretch capacity of belts represents the outer limit of any workable prior art transmission, and those of ordinary skill in the art would consider even this ten percent stretch capacity undesirably high.
It is, thus, apparent that there is a need for improved automatic transmission designs to improve upon existing technology and to provide alternatives for designers in the many industries using automatic transmissions. Preferably, such improved designs would be simple, inexpensive and provide high reliability.